MicroRNAs are emerging as important regulators of cancer-related processes. Our studies show that microRNA-9 (miR-9) is downregulated in human ovarian cancer relative to normal ovary, and overexpression of miR-9 suppresses cell growth in vitro. Furthermore, the 3'-UTR of NF-kappaB1 mRNA is found to be regulated directly by miR-9, demonstrating that NF-kappaB1 is a functionally important target of miR-9 in ovarian cancer cells. When miR-9 is overexpressed in ovarian cancer cells, the mRNA and protein levels of NF-kappaB1 are both suppressed, whereas inhibition of miR-9 results in an increase in the NF-kappaB1 expression level. Ovarian cancer tissues display significantly low expression of miR-9 and a high level of NF-kappaB1 compared with normal tissues, indicating that regulation of NF-kappaB1 by miR-9 is an important mechanism for miR-9 to inhibit ovarian cancer proliferation.